The spine is composed of both rigid and flexible elements which form a complex structure that can readily accommodate a wide range of motions and adjust to a wide range of loads. Unfortunately, like any complex physiological structure, the spine is also vulnerable to disease, injury, and congenital deficiencies, all of which can cause defects to the spine and, in particular, to the vertebral body and intervertebral discs. Spinal disease, injury, and deformity may have a disastrous impact on patient well being, ranging from acute pain to chronic debilitating pain and, in the most severe cases, partial or complete paralysis.
Some of the most common pathologies of spinal defects include fractured, diseased, or decayed vertebral bodies; torn or stretched ligaments; and damaged or diseased intervertebral discs.
Common treatments for damaged, diseased, or defective vertebrae include joining or fusing fractured bone segments or portions together to stabilize the affected parts and removing the affected vertebrae, either in part or in whole. Classically, the damaged disc is excised, the adjacent vertebrae are mechanically joined together, and oftentimes bone is grafted into the region, particularly in the disc space between the two vertebrae, to promote fusion of the adjacent vertebrae. The vertebrae can be mechanically joined using a prosthetic device such as a bone plate that is attached to the adjacent vertebrae with bone screws. The bone plate eliminates disparate motion between the two bone portions to allow arthrodesis.
It is known that for load bearing bone members, stronger, denser bone tissue results when new bone growth occurs under pressure and that the risk of a weakened juncture or pseudoarthrodesis increases when a prosthetic device stress shields new bone growth. The problem arises of when and how much pressure or force to apply to develop a strong junction between the bone portions. The bone portions should be secured and supported during initial bone growth. However, the optimum support necessary for desired bone growth may vary over time as the bony juncture or bridge develops between the bone portions.
Similarly, torn and/or structural ligaments can be treated by initially securing/immobilizing the ligaments. This can be accomplished using internal and/or external prosthetic devices to augment or replace the stability lost as a result of the damaged ligaments. Further, the treated ligaments can be susceptible to repeated injury. Consequently, it may be desirable to augment the treated ligament by implanting a prosthesis or device that allows limited movement of the affected ligaments, i.e., stretching and rotation of the ligaments. Current treatment methods do not allow for an implanted device to initially secure/immobilize the ligaments and then allow limited movement of the same without a subsequent surgical revisitation.
In light of the above, there is a continuing need for devices and treatments that stabilize and support damaged bone tissue, bony structures, and connecting tissue and provide variable loads to growing bone as well as a measure of flexible support to injury- or disease-prone bones and connecting tissue. The present invention addresses this need and provides other benefits and advantages in a novel and nonobvious manner.